Manufacturers of microelectronic devices require process gases having extremely low levels of entrained particulate contamination. Small quantities of microscopic contamination can damage micro circuits during the manufacturing process. Therefore, the concentration of contaminant particles in such gases must be tightly controlled. Modern filters are able to remove particulate contaminants in process gases with an extremely high efficiency. However, the complete assurance of contamination control also requires verification of gas cleanliness. An accurate technique for detecting microscopic particles in filtered gases must be available. Sensitive instruments capable of detecting particles as small as about 0.003 micrometer are readily available for inert process gases, such as nitrogen. However, particulate control must also be maintained in reactive gases, such as hydrogen and oxygen, which are used to manufacture various microcircuitries.
Instruments designed for inert gas particle counting cannot be used for 100% reactive gases, such as hydrogen or oxygen. Only a limited number of less sensitive instruments are available for hydrogen and oxygen type reactive gas particle counting service. Particles as small as about 0.1 micrometer can be measured in hydrogen or oxygen using, for example, certain particle counters manufactured by Particle Measuring Systems, Inc. of Boulder, Colo. These particle counters which belong to the family of so called laser spectrometers, have been especially designed for safety in the presence of 100% reactive sample gas streams and have been tested for proper calibration using these gases. However, there is presently no commercially available instrument to measure particles smaller than about 0.1 micrometer in oxygen or hydrogen.
Turbulent mixing diluters for diluting particulate containing reactive gases with inert diluent gases are known. However, various drawbacks in turbulent mixed diluters include the requirement for taking contamination measurements of both the dilution inert gas and the diluted particle-containing gas. In addition, microscopic particles which are of interest to the electronics industry are potentially lost on vessel walls in turbulent mixing diluters, which can throw off the detection of particles and thus make the measurement inaccurate. Such disadvantages for turbulent mixing vessels as diluters for reactive gas particle counting make them impractical for ultra clean low particle concentration gas applications requiring highly accurate measurements.
Other diluters are known in the industry such as set forth in U.S. Pat. No. 4,684,251 wherein a spectrometer with a sample diluter comprises a sample liquid line 19 which is diluted in vessel 1 with a diluent from vessel 10 which then flows in laminar flow through curved tubing 5 to an atomizer 6, after which the sample is burned and subject to spectrometric analysis. The curved nature of conduit 5 and the destruction of the sample after atomization makes this patent disclosing laminar flow inapplicable to enhancement of accurate particle counting.
U.S. Pat. No. 5,058,440 discloses a gas sampling dilution tunnel used for sampling the exhaust of an internal combustion engine particularly for particulates. A sampling probe 56 is introduced into a flow of particle-containing gas and removes a slip stream which is then mixed with diluent air introduced tangentially to the flow of the sample through a porous perforated tube 14 before being passed through a valve at a right angle where particles are adhered on filter 68. The tangential flow of diluent air with the concomitant addition of any particles in the diluent air to those in the sample gas and the angled flow path leading to a filter media, as well as retention of particles on the filter for counting makes this apparatus and procedure inappropriate for ultrafine, ultra low concentration particle detection in process gases used for the micro electronics industry.
U.S. Pat. No. 5,026,155 discloses a condensation nucleus counting device and method wherein a particle-containing gas in line 10 is contacted at capillary 22 with a working fluid vapor containing gas at a temperature such that the working fluid vapor condenses on the particles in conduit 26 and enlarged droplets surrounding the nucleating particles can be detected photometrically at the detection station 34 to provide an accurate count of particle contamination in the sampled gas stream. However, the working fluids are noted to be inclusive of alcohols and various organic materials which along with the materials of construction of the apparatus may be incompatible with various reactive gases, such as hydrogen and high purity oxygen.
The disadvantages of dilution and materials incompatibility of the prior art are overcome by the present invention, which is set forth in greater detail below.